dd1 had hers at a local hairdressers when she was about 1 - had a mop of hair that needed tidying up. Dd2 is now 18 months and nowhere near needing a hair cut.
i recently cut dd1's hair - but she has it all one length and I did a wet cut after her hair had been washed - jsut started with one area and cut slowly across checking as I went it was straight. Dh entertained her and kept her still for me..... probably not perfectly straight but her hair grows so thick and would be sooo long without regular cutting ( she is almost 4 now) Most of the time it is up in a ponytail or bunches to stop it annoying her anyway.